System Tasks for File Output & Strobing

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Introduction

There are tasks and functions that are used to generate input and output during simulation. Their names begin with a dollar sign ($). The synthesis tools parse and ignore system functions, and hence can be included even in synthesizable models.

We will discuss system tasks for file output & strobing in this PPT.

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File Output

Output from Verilog normally goes to the standard output and the file verilog.log.

It is possible to redirect the output of Verilog to a chosen file.

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Opening a File - 1

A file can be opened with the system task $fopen.

Usage: $fopen("<name_of_file>” ) ;Usage: <file_handle> =

$fopen("<name_of_file>");

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Opening a File - 2

• The task $fopen returns a 32-bit value called a multichannel descriptor. Only one bit is set in a multichannel descriptor.

• The standard output has a multichannel descriptor with the least significant bit (bit 0) set. Standard output is also called

channel 0. The standard output is always open.• Each successive call to $fopen opens a new channel

and returns a 32-bit descriptor with bit 1 set, bit 2 set, and so on, up to bit 31 set.

• The channel number corresponds to the individual bit set in the multichannel descriptor. Next slide illustrates the use of file descriptors.

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Opening a File - 3

//Multichannel descriptor

integer handle1, handle2, handle3; //integers are 32-bit values

//standard output is open; descriptor = 32'h0000_0001 (bit 0 set)

initial

begin

handle1 = $fopen("file1.out"); //handle1=32'h0000_0002 (bit 1 set)

handle2 = $fopen("file2.out"); //handle2=32'h0000_0004 (bit 2 set)

handle3 = $fopen("file3.out"); //handle3=32'h0000_0008 (bit 3 set)

end

The advantage of multichannel descriptors is that it is possible to selectively write to multiple files at the same time.

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Writing to Files - 1

The system tasks $fdisplay, $fmonitor, $fwrite, and $fstrobe are used to write to files. Note that these tasks are similar in syntax to regular system tasks $display, $monitor, etc., but they provide the additional capability of writing to files.

Usage:

$fdisplay(<file_descriptor>, pl, p2,..., pn);$fmonitor(<file_descriptor>, pl, p2,..., pn);

pl, p2, ... , pn can be variables, signal names, or quoted strings.

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Writing to Files - 2

A file_descriptor is a multichannel descriptor that can be a file handle or a bitwise combination of file handles.

Verilog will write the output to all files that have a 1 associated with them in the file descriptor.

We will use the file descriptors defined in Slide-6 and illustrate the use of the $fdisplay and $fmonitor tasks.

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Writing to Files - 3

//All handles defined in Slide-6

//Writing to files

integer desc1, desc2, desc3; //three file descriptors

initial

begin

desc1 = handle1 | 1; //bitwise or; desc1 = 32'h0000_0003

$fdisplay(desc1, "Display 1") ; //write to files file1.out & std.out

desc2 = handle2 | handle1; //desc2 = 32'h0000_0006

$fdisplay(desc2, "Display 2") ; //write to files file2.out & file1.out

desc3 = handle3 ; //desc3 = 32'h0000_0008

$fdisplay(desc3, "Display 3") ; //write to file file3.out only

end

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Closing Files

Files can be closed with the system task $fclose.

Usage: $fclose(<file_handle>);

//Closing Files$fclose(handle1) ;

A file cannot be written to once it is closed. The corresponding bit in the multichannel descriptor is set to 0. The next $fopen call can reuse the bit.

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Strobing

Strobing is done with the system task keyword $strobe.

This task is very similar to the $display task except for a slight difference. If many other statements are executed in the same time unit as the $display task, the order in which the statements and the $display task are executed is nondeterministic.

If $strobe is used, it is always executed after all other assignment statements in the same time unit have executed.

Thus, $strobe provides a synchronization mechanism to ensure that data is displayed only after all other assignment statements, which change the data in that time step, have executed.

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Strobing - Example

//Strobing

always @(posedge clock)

begina = b;c = d;

end

always @(posedge clock)$strobe("Displaying a=%b, c=%b", a, c); //display values at

posedge

The values at positive edge of clock will be displayed only after statements a =b and c =d execute. If $display was used, $display might execute before statements a =b and c =d, thus displaying different values.

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